Known rotary blades (sometimes referred to as rotary vanes) for an axial flow fan are shaped to be swept forward in a rotation direction and inclined forward to a suction side to reduce noise. With such rotary blades, a shape of a bellmouth has been employed that is semi-open to overlap portions of the rotary blades at trailing edges thereof (hereinafter referred to as a “semi-open bellmouth”). Reducing noise has been thus achieved by configuring the shape of the bellmouth or the positional relationship between the blades and the bellmouth.
To further reduce the noise, a shape of the rotary blade has been recently proposed that can reduce interference caused by a blade tip vortex. In one such proposed shape, the blade is bent toward an upstream side of a flow of air at a blade outer peripheral portion of the blade. This is to address a leakage flow around the blade outer peripheral portion from the pressure surface side to the suction surface side, and a blade tip vortex generated on the blade suction surface. The leakage flow is generated when the blade is rotated due to a pressure difference between a pressure surface and a suction surface of the rotary blade, and the blade tip vortex is generated due to this leakage flow. Interference of the leakage flow and the blade tip vortex with a vane surface, an adjacent blade, and the bellmouth is a cause of increased noise.
An axial flow fan to be used for a ventilation fan, an air-conditioning apparatus, a cooling fan, or other devices, is rarely equipped solely with rotary blades; a bellmouth is arranged around the rotary blades to rectify a flow and increase a pressure. Therefore, not only the rotary blade is relevant to air blowing and noise characteristics, but also do the other factors, such as the shape of the bellmouth, the positional relationship between the rotary blades and the bellmouth.
A known axial flow fan includes a propeller fan having a plurality of blades formed on an outer periphery of a hub, and a fan guide. A dihedral angle on a mean flow surface of the blade is set to approximately 60 degrees. The fan guide is formed into a tubular shape, and a length of the fan guide in an axial direction is set to 0.8H or more of a height H of the blade. A suction-side end portion of the fan guide is shifted toward a discharge side from suction-side end portions of the blades, and an amount U of the shift is set to satisfy a relationship of 0.3H≤U≤0.5H (see, for example, Patent Literature 1).
Further, there is known an axial flow fan including a propeller fan configured to be rotated by a drive source, a tubular air introducing portion covering the propeller fan while securing a predetermined size of space behind the propeller fan, and a shroud formed continuously to the air introducing portion to be opened on a front side of the propeller fan so as to introduce air in a wide range on the front side of the propeller fan to the air introducing portion. The opening portion of the shroud is obliquely shaped and narrowed to the air introducing portion. Each of the blades of the propeller fan is formed in such an inclined manner as to be swept forward in a rotation direction, whereas a leading edge portion of each of the blades formed to be approximately perpendicular to the rotation axis of the propeller fan. A forward end side of the propeller fan in the direction of the rotation axis is arranged to be swept forward from a boundary between the air introducing portion and the opening portion toward the opening portion by a predetermined amount (see, for example, Patent Literature 2).
Further, there is known a blower device including a propeller fan having a plurality of blades being formed on an outer peripheral surface of a hub as a rotation center and each having a thick blade shape as typified by an airfoil blade, a bellmouth being located on a radially outer side with respect to the propeller fan and partitioning a suction region and a discharge region, and a fan guard located on a discharge side with respect to the propeller fan. The bellmouth includes a suction-side arc portion located on a suction side, a discharge-side arc portion located on the discharge side, and a cylindrical portion located between the discharge-side arc portion and the suction-side arc portion. When a height of the bellmouth in an axial direction at a portion overlapping with an outer peripheral portion of each of the blades is represented by H1, and a height of the outer peripheral portion of each of the blades is represented by H0, those parameters are set so as to fall within a range of H1/H0=0.40 to 0.65 (see, for example, Patent Literature 3).
As disclosed in Patent Literatures 1 to 3, as for the related-art rotary blades, consideration is made on the axial flow fan including the semi-open bellmouth that overlaps with the bellmouth in a range from the chord center to the blade trailing edge, and the axial flow fan constructed such that most part of each of the rotary blades is accommodated in a bellmouth air channel.
Further, as for the propeller fan having the shape in which the blade outer peripheral portion is bent in the upstream direction of the flow of air, an optimal positional relationship between the rotary blades and the bellmouth is proposed as well as a shape of the bellmouth.
For example, there is known an axial flow fan including a hub being a rotation center, a plurality of vanes being formed on an outer peripheral surface of the hub so that outer peripheral ends of a leading edge and a trailing edge are located on a front side in a rotation direction, and a bellmouth being arranged so as to surround outer peripheries of the plurality of vanes and including an air inlet-side first round surface portion, a cylindrical portion having a predetermined width and being located on a downstream side with respect to the inlet-side first round surface portion, and an air outlet-side second round surface portion located on a downstream side with respect to the cylindrical portion. An outer peripheral end portion of each of the vanes is inclined toward the air inlet-side. The outer peripheral end of the inclined trailing edge portion of each of the vanes is located at an air outlet-side end portion of the cylindrical portion of the bellmouth. The trailing edge portion of each of the vanes at a part other than the inclined part is located at an air outlet-side end portion of the second round surface portion of the bellmouth (see, for example, Patent Literature 4).
Further, in order to obtain an axial flow fan in which blade outer peripheral portions are each bent in an upstream direction of a flow of air and most part of each of blades is accommodated in a bellmouth air channel, and in which noise caused by a blade tip vortex is small and the degree of decrease in air blowing performance is small, there is also proposed an axial flow fan including a bellmouth air channel gradually reduced in diameter from a large-diameter air inlet side to an air outlet side, in which most part of each of the blades is accommodated in the bellmouth air channel (see, for example, Patent Literature 5).
Further, there is also proposed an axial flow fan including a boss configured to be rotated about an axial center, and a plurality of rotary blades arranged on an outer peripheral portion of the boss. The rotary blades are each formed so that a chord center line connecting chord center points from an inner peripheral end to an outer peripheral end of the rotary blade is curved to protrude toward a downstream side of a flow of air in an entire region of the rotary blade in a radial direction (see, for example, Patent Literature 6).